A simplified diagrammatic representation of a disk drive, generally designated as 10, is illustrated in FIG. 1. The disk drive 10 includes a data storage disk 12 that is rotated by a spindle motor 14. The spindle motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a read/write transducer 20 (or head) mounted to a flexure arm 22 which is attached to an actuator arm 24 that can rotate about a pivot bearing assembly 26. The actuator arm assembly 18 also includes a voice coil motor (VCM) 28 which moves the transducer 20 relative to tracks defined on the disk 12. The spindle motor 14, VCM 28, and transducer 20 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. Although a single disk 12 is illustrated in FIG. 1, the disk drive 10 may instead include a plurality of disks with a transducer adjacent to each disk storage surface.
FIG. 2 is an exemplary top view of the disk 12. Data is stored on the disk 12 within a number of concentric tracks 40 (or cylinders). Each track is divided into a plurality of radially extending sectors 42 of the disk 12. Each sector 42 is further divided into a servo sector 44 and a data sector 46. Information in the servo sectors 44 is used to, among other things, accurately position the transducer 20 so that user data can be properly written onto and read from the data sectors 46.
FIG. 3 illustrates exemplary servo information 73 that can be stored in each of the servo sectors 44. The servo information 73 can include a DC erase field 731, a preamble field 732, a servo address mark (SAM) field 733, a track number field indicated by its least significant bits (LSBs) 734, a spoke number field 735, an entire track number field 736 which is recorded in at least one of the servo sectors 44, and a servo burst field 737 of circumferentially staggered radially offset servo bursts (e.g., A, B, C, D servo bursts). FIG. 3 also illustrates an exemplary length of each of the fields in units of servo bit times (T) (i.e., field 731 may have a length of 40 bit times (40 T)). The DC erase field 731 can indicate to the circuits 30 the onset of a servo sector 44. The preamble field 732 can be written with a 2 T repeating pattern (e.g., “−−++ −−++ −−++”) of a defined length. The preamble 732 may be used by timing and gain loops in the circuits 30 to establish a gain and phase lock relationship for sampling the analog signal that is generated when reading the servo information through the transducer 20.
A servo controller in the electronic circuits 30 determines the position of the transducer 20 relative to the tracks 40 in response to the servo information read from the servo sectors 44. The servo controller uses the determined position to move the transducer 20 from an initial track to a target track (i.e., seek operation), and to maintained the transducer 20 aligned with the target track while data is read/written on the disk 12 (i.e., track following operation). During a seek operation, the track addresses are used as coarse positioning information to estimate the position of the transducer 40 as it is moved to the target track. During track following, the servo bursts are used as fine positioning information to precisely align the transducer 40 over the selected track.
While seeking the transducer 20 across tracks, the transducer 20 can read servo information from several radially adjacent servo tracks within a same one of the servo sectors 44. Consequently, the signal that is read while crossing a servo sector can include a combination of servo information from different servo tracks. As the seek speed increases and/or as servo information density increases, the transducer 20 can read from a greater number of radially adjacent servo tracks in the same servo sector. In an attempt to allow the servo controller to distinguish between the combined servo information signal, the track addresses can be encoded as Gray-coded addresses in which the encoded addresses of any two radially adjacent tracks differ from each other by only in a single bit position. Accordingly, when the transducer 20 scans across two servo tracks the ambiguity after decoding the address is one track. A positioning error of one track can be acceptable while seeking.
Also, because the transducer 20 can read from several different servo tracks in a servo sector while scanning, an attempt is made when formatting the disk 12 to write the servo information so that it is phase aligned between radially adjacent servo tracks. However, the servo-track writers (“STW”) used to format disks have limited accuracy, and they typically allow at least some amount of phase misalignment to occur between servo information in radially adjacent servo tracks. This phase misalignment is known as radial incoherence.